Surround Sound: High Tech Meets the Beast Within Us The Mathematics of Dolby's Mono Surround Sound

Sidebar: The Mathematics of Dolby's Mono Surround Sound

Even though it may come from two loudspeakers, the surround signal on Dolby-encoded recordings is a single channel—monophonic. To encode it into the main stereo channels, it is subtracted from each channel by inverting its electrical polarity and then adding it to the front-channel signals. (Adding a minus to a plus subtracts it.)

But it's not quite that simple. Since the surround effects often contain the same frequencies as front-channel sounds, simply subtracting the surround from the fronts would impair the sound in front. So a compromise is struck. Instead of reversing the surround's polarity—that is, changing its phase by 180 degrees—the surround signal is split into two identical channels. Each is "phase-shifted" by half that amount, or 90 degrees, and in opposite directions. One is phased +90 degrees relative to the fronts, the other is phased –90 degrees relative to the fronts.

They are, thus, 180 degrees out of phase relative to each other, but only halfway out relative to the fronts, which drastically reduces their detrimental effect on the front signals. Instead of taking a deep bite out of the front sounds, they just take a nibble. To decode the surround signal, the process is reversed. The left and right channels are then split off to yield two pairs of identical left and right signals. One of these pairs is then subtracted from the other, and what's left is the original surround signal, more or less intact.—J. Gordon Holt

Kal Rubinson's picture

All that is so much more complex than modern discrete multichannel and, at the same time, so much less efficient or successful in creating convincing surround. It takes the increased data bandwidth that we enjoy today to make real multichannel possible.

eriks's picture

Hey Kal,

I have a background in this subject as I worked for one of Dolby's competitors when Dolby Surround was still being used.

Honestly, the recorded information on Dolby Surround film was _really_ good. The issues with it was not the lack of discrete channels, or bandwidth but rather the steering matrix was far too interested in effects, and how noisy Dolby processors at the time were. I got to listen to prototype discrete decoders that you could adjust this on and let me tell you, the immersion factor could be glorious by just dialing all that down and letting the tracks talk without being constantly micromanaged by the steering matrix.

I'm not saying it's as good as multi-track magnetic, or modern discrete digital tracks. I'm just saying that there was a lot more sound and music in those tracks than most people realize. That it sounded so campy was a choice made by Dolby which recording engineers could not get around.



Kal Rubinson's picture

I will not dispute that but the result, as seen in the open market, was that Dolby Surround had negligible and transient effect on music recordings.

Prior to the appearance of discrete multichannel, I played around with a Meridian system and the experience established my interest in "surround sound" but it was not sufficient to get me to commit to it. Later discrete, albeit lossy, media were a move in the right direction. However, it was not until the arrival of discrete, lossless formats that significant numbers of music recordings became available to justify (for me and others) rebuilding my system from stereo to multichannel.

eriks's picture

If you mean, should you use Dolby Surround for music, I have to agree with you. It was always a system centered on effects, not transparency. I only meant to bemoan how little of the sound on the tracks we would hear because of it.

Have you heard the Neo6 modes for 2 channel music though?

If you ever notice that instruments are brighter at the sides, but less so when in the center, Neo6 fixes this, with none of the downsides of Dolby Surround.

Kal Rubinson's picture

Thanks but I have no real interest in movie sound or HT. In what is probably the reverse of the experience of many, my HT setup is the by-product of my multichannel music interests.

eriks's picture

I understand, but do try Neo6 Music mode for some 2 channel listening. :) It may help you use your center channel more often.

Bogolu Haranath's picture

Have you tried or know about DTS headphone X? ...... Is it supposed to make binaural sound to multi channel surround sound? .........

Kal Rubinson's picture

eriks said:
I understand, but do try Neo6 Music mode for some 2 channel listening. :) It may help you use your center channel more often.

I have but it didn't stick (i.e., not good enough for continued use. OTOH, I do use Auro 2D/3D occasionally.

Bogolu Haranath's picture

My question was directed to eriks ........ Apparently there was a mix up ....... If eriks is reading this, may be he could tell us some thing about DTS headphone X :-) ............

eriks's picture

As you may know, in between Dolby Surround and Dolby Digital there where discrete magnetic tracks for film, both 35mm and 70mm. Not many movies were released in this format, as it required new equipment to read it and the oxide would wear off, and it produced a lot of dust.

The major benefits were noise, extension but also they did away with the DS matrix, so this format was used for musically oriented films like Yentl, The Sound of Music, Amadeus, but also the occasional weird one like The Natural and Caligula. Maybe we should get a BD of Caligula and see what extraordinary musical content they had? :D

tonykaz's picture

...came closest ( am I wrong ? ) to describing a Sonic Holidome experience of replicating "being there", using available gear, making home audio hobbyist's participation a possibility, for the first time that I'm aware of.

Otherwise, isn't 5.1 and it's variations a "sound effects" gimmick?

Mr.KR is the first Serious Audiophile to attempt elevating the genre ( as far as I'm aware of) , there must be something there to enjoy, something otherwise missing.

Sitting in Central Park NY and hearing sounds from all around me, I don't know if I'd like to bring that Sonic Landscape into my home but having the option is thought provoking.

It might even be ADDICTIVE. Phew!!

I'm stay'n tuned.

Thanks for all the reporting on this.

Tony in Michigan

Kal Rubinson's picture

Tony said: Sitting in Central Park NY and hearing sounds from all around me.......

Really? I'm only 4 blocks away!

tonykaz's picture

I was using Central Park figuratively, as a visualization tool, because it has Central as part of the meaning I'm struggling to convey.

I've stayed at the Mark Hotel on the edge of the Park.

I rather love Central Park, NY. if I'm in NY.

Tony in Ice Storming Michigan

Kal Rubinson's picture

Tony said: I've stayed at the Mark Hotel on the edge of the Park.

High tea at the Mark. A delight.

Robin Landseadel's picture

Best "surround" experience—standing the middle of the Berkeley Symphony Orchestra during a rehearsal of Olivier Messiaen's "Éclairs sur l'Au-Dela" during a passage where the woodwinds were performing birdsong—an orchestral aviary.

ednazarko's picture

I was a professional brass player - mostly trombones and tubas - for years, and I really miss how music sounds from inside. Orchestras, where because I sat in the back, my soundstage was reversed from the audience's soundstage. And big bands, where I loved sitting inside back row. The "mix" I remember is also quite different from what comes on recorded music. With classical music I always want the winds mixed louder.

The simulated surround sound function on my pre-pro can get me close to happy from my normal listening spot. I think my taste for dipole plus cardioid speakers (Gradient Revolutions), or open baffle speakers is because the sound envelops more. I find myself dragging a chair in quite close for stereo, or smack in the middle for surround.

I've had some absolutely spooky surround sound experiences with binaural recordings and open back headphones. First time I listened to one album I startled a few times (without thinking whipped around to look) because I heard someone sniffing who seemed precisely placed behind me. I have a handful of binaural recordings, that through the right headphones, beat my 5.1 system for that spooky feeling of being present.

Bogolu Haranath's picture

Some of us also listen to headphones, both closed back and open back ......... May be you could mention few of those binaural recordings? ....... Some of us may be interested :-) ........

ednazarko's picture

Chesky Records (he owns HD Tracks) does a LOT of binaural. Search and find. I recommend Wyclyffe Gordon "Dreams of New Orleans", Amber Rubarth's "Sessions from the 17th Ward", Macy Gray's "Stripped"... and Dunun Kann, drummers from Guinea.

The first binaural recording I ever heard was a poetry reading (via B&W record release.) I had my CIEMs in, on a plane waiting to take off, and that's when I had my near-neck-breaking moment of "someone's got a cold right behind me." Seats in the next two rows were empty... and I had CIEMs on so I'd have never heard that... then it happened again, and "behind me" was the fuselage and window. The effect is there with any headphone, but for me is strongest on open backed.

Chesky records in interesting places - like a couple of churches with great acoustics, so the sense of space is massive, unlike how headphones can present quite a small image. The knock on binaural is that they don't sound great through speakers, but I don't find that to be true. The space and aliveness on Macy Gray's album is just as spooky through speakers.

dc_bruce's picture

stuck on the wall behind you.

It's easy to dismiss these, but Bose was/is on to the central problem of multichannel sound, whether accompanied by video or not. That is, what do you do with all that "stuff" in the room?

Perhaps one reason for the decline of "specialist audio" is its increasing demands on the listening room.

Consider, in the "hi-fi" era, mono reproduction required only one speaker that, perhaps, could be shoved off into an unused corner. And in the early stereo era, before we got all excited about "imaging," "soundstage" and the like, many speakers were just shoved into a bookshelf, or stood on the floor (in a corner!).

Today, it's just about impossible to have a "serious" stereo that doesn't dominate the room, like a sacrificial altar to the audio gods. Even small speakers that used to be called "bookshelf" have been re-labelled "stand mount" speakers that demand to be set on expensive stands, well out into the room.

I think a lot of us, who don't want the stereo to be relegated to the basement and who don't want to cut ourselves off from our family, have come to accept some sonic compromises. And, while the sonic benefits of multichannel are indisputable, for lots of folks those benefits don't outweigh the costs -- financial and otherwise.

ednazarko's picture

My 5.1 system is nowhere near set up the way I'd like, but I was told that I could only have the rear speakers where I wanted them if I wanted a different wife. Sigh. Still, it's good enough for movies, and for the occasional 5.1 mixed album. When she's got a business trip I pull the speakers out to where I want them and binge-listen.

cozmoz's picture

Wonderful details, but the only thing I could not understand the relation between bone conduction and surround sound. How we are going to merge both? One is physical and other one is more extrinsic in nature.

cozmoz's picture

Wonderful details, but the only thing I could not understand the relation between bone conduction and surround sound. How we are going to merge both? One is physical and other one is more extrinsic in nature.